1. Technical Field of the Invention
The present invention relates to cryogen gas purifiers for removing impurities from a supply of cryogen gas, and more particularly to helium gas purifiers configured to de-sublimate impurities by cryo-condensation that, optionally, utilize filter means for further facilitating removal of such impurities. The invention further includes methods for purging such impurities or otherwise regenerating the purifiers for continuing operation.
2. Description of the Related Art
Cryogen gases are in high demand for their application in refrigeration and cooling technologies, as well as other applications. For example, helium gas, among other cryogen gases, is often used in a variety of medical and scientific equipment, including magnetic resonance imaging (MRI), material analysis devices, and other equipment. To achieve liquid-phase helium for use with refrigeration technologies, gas-phase helium is generally liquefied within a gas liquefier by cooling the gas to a point of liquefaction. The liquid-phase helium is then evaporated to produce a flow of gas-phase helium for cooling material samples, superconducting magnets, or other materials or components.
Due to the scarcity of helium, as well as the high consumption of the cryogen gas, there is much interest in the recovery of the evaporated liquid from medical and scientific equipment that is afterwards purified and liquefied to be used again. For example, apparatuses such as magneto encephalography (MEG), nuclear magnetic resonance (NMR), physical properties measurement systems (PPMS), and magnetic properties measurement systems (MPMS), among others, can consume from 1 to 10 L/day of liquid helium.
When the overall consumption of a facility, such as a hospital or scientific laboratory, is below 100 L/day, conventional helium recovery and liquefaction practices (i.e., those based on the pioneering work of Professor Samuel C. Collins and derived technologies), are too big and inefficient due to a significant amount of the evaporated helium that is lost into the atmosphere. As an alternative, there is presently an emerging commercially-available technology, based on cryocoolers, for recovery and liquefaction at the small scale (<100 L/day), which adapts liquefaction to consumption and maintains the liquid produced without losses until a transfer to the liquid helium user equipment is needed. Exemplary systems that are currently available include helium liquefiers produced by Quantum Design of San Diego, Calif.; Cryomech of Syracuse, N.Y.; and Quantum Technology of Blaine, Wash. Such technology is proving to be sufficient for helium recovery of single, as well as for multiple, medical and scientific instruments so that helium losses could be minimized.
While the liquefaction technology of small scale helium recovery systems based on cryocoolers works properly when using commercial-grade, high purity gas where total impurities concentrations are less than 1 in volume ppm, the efficiency is immediately lost when using recovered gas having impurity concentrations greater than 1 ppm in volume. For the recovery of helium from single or multiple medical and scientific instruments, however, the necessary purification technology prior to liquefaction (i.e., producing pure gas at a level of <<1 ppm total impurity content) is not efficient enough.
In order to provide sufficiently purified gas to a liquid helium plant or system, there is thus typically deployed a gas purifier that is operative to remove impurities in the in-coming feed gas. In this regard, gas purification is a separation process whose sole purpose is removal from the process gas of unwanted traces, or small amounts of contaminants, termed impurities. After purification, the purified cryogen gas is removed (e.g., transferred to liquefier), the separated contaminants are discarded and the device used for purification is regenerated for re-use.
Currently, three different gas purification methods are being used in conjunction with Small Scale Helium recovery plants. Those methods are as follows:
1. Chemical Gas Adsorption: The gaseous helium mixture is brought in contact with a solid product, the getter, at high temperatures. The impurities (mainly N2 and O2 for the case of recovered helium) are eliminated by a chemical reaction with the getter to a level of 10−3 ppm, independently of their concentration in the input gas. The main limitation with this methodology is the maximum amount of impurities of the recovered gas at the input of the device, which has to be maintained below 10 ppm in volume, to avoid excessive heat generated by the very high exothermic chemical reactions with the impurities. However, most of the recovery systems, especially those using gasbags, in a best case scenario, have a minimum volume ratio concentration of 1.5×10−4 in total. Therefore, this technique cannot be applied for purposes of the present invention. This technique also produces an undesirable increase of pressure drop as a function of the amount of reacted product, reaching several bar even at low flow rates (<10 sL/min) that further makes such method impractical for low-pressure recovery systems (e.g., <2 bar).
2. Cryogenic Gas Adsorption: The gaseous helium mixture is brought into contact with a material that has a high surface to volume ratio, then cooled to low temperatures of around 80 K using liquid nitrogen as a cooling agent. Since this is a surface effect, big volume ratios of the adsorption material versus the impurities present in the incoming gas are needed in order to be effective. When the adsorption material gets saturated, the system has to be heated at high temperature and regenerated by pumping. Therefore, twin systems are necessary for continuous operation, as well as liquid nitrogen refill operations to provide the required subsequent cooling. Moreover, the impurities concentration of the output gas often depends on the impurities concentration at the input. In this regard, output concentration levels below 10−5 are not easily achievable.
3. Cryo-condensation: Purification by cryo-condensation is accomplished by bringing in a phase change of the impurities sought to be removed. Cooling the incoming feed gas by means of refrigeration in a device at low temperatures (T<30 K for the case of nitrogen in helium) facilitates condensation of readily condensable impurities. As soon as the mixture gets supersaturated, the corresponding impurity de-sublimates and coats the cold surfaces of the container and/or precipitates out from the feed gas. That is, as soon as the mixture temperature reaches the value at which the equilibrium vapor pressure of the impurity is less than the impurity partial pressure in the mixture, the impurity starts to de-sublimate. Total N2 and O2 output impurity levels of 0.1 ppm or less in helium, when working at low pressures (<2 bar) and low temperatures (<30 K), are easily achievable. Even though there are already some advances on this kind of method using a device with a two stage cryocooler, continuous operation during long periods (months) while keeping operational flow rates of the order of 30 L/min in the process gas are still a challenge.
An exemplary prior art system for removing impurities from a helium feed gas is described in U.S. patent application Ser. No. 13/937,186, entitled CRYOCOOLER-BASED GAS SCRUBBER, filed on Jul. 8, 2013, which is based on cryo-condensation and/or coalescence of impurities on a very high effective coalescent/de-sublimation surface area material. The disclosed system uses a purifier cartridge filled with glass wool, occupying almost the entire Dewar impurities storage region, in order to get less than 5×10−6 of N2 with a maximum flow rate of 25 L/min. This limitation is due to the fact that as soon as the cooling device (a two stage refrigerator coldhead) and the surface of the corresponding output gas counter flow heat exchanger are coated by frost, not all the impurities are frozen and trapped on the deep cooling region but rather are forced to “coalesce” in contact with a high surface material, like glass wool that is densely packed inside a cartridge occupying the impurities storage volume. The main drawbacks of that system are as follows:
1. The impurities storage effective volume is only a small fraction of the Dewar volume, typically 10%, and thus can only provide a limited impurity storage capacity.
2. Both the Dewar neck and the Dewar belly, having small passages for the input gas flow, are easily blocked by frost. To minimize this drawback, a minimum flow back to the recovery system of around 5 L/min has to be maintained at all times, even when the liquefiers are not demanding any gas flow.
3. Periodic regenerations are required, typically once a week, which necessitates heating up the whole system (i.e., coldhead, heat exchanger, cartridge, Dewar belly) to above 120-150 K, and evacuating it completely.
4. The densely-packed filter cartridge represents a thermal load that makes the cool-down process after regeneration take a minimum of 3-6 hours, thus interrupting the liquefaction process during that additional time.
Accordingly, there is a substantial need in the art for methods and devices for purifying a process gas mixture that is exceptionally effective and efficient in removing impurities from the gas mixture that is also operative to provide a large volume to store impurities and can further eliminate the need for frequent regeneration processing. Along those lines, there is a need for such a system and method, as well as a method to efficiently regenerate such a system to thus enable cryogen gas purification to operate continuously without interrupting the supply of purified gas for prolonged periods of time (e.g., months). There is especially a need for such a system that can accomplish such objectives that is specifically tailored to helium recovery systems whereby adequate volumes of cryogen gas can be purified in a highly effective and economical manner.